Flexible structures, such as aircraft, are potentially prone to wind-induced vibration due to a range of fluid-dynamic effects including vortex shedding, turbulent buffering, galloping and flutter. In many cases the vibrations are limited in magnitude, resulting in increased loads and occupant comfort issues. These effects can be generally taken into account in design. In other cases, however, instabilities may occur that can result in excessive vibration and consequently failure, which should be avoided within the wind speed range likely to be experienced by the structure. To assess the significance of wind-induced dynamics in structural design, specialized studies are required. These specialized studies may include dynamic and modal testing.
One objective of dynamic and modal testing is to measure the dynamic characteristics of a particular structure or test article to confirm and validate a finite element model before operation and production. Ground vibration testing (GVT) is a particular method of measuring the structural dynamic properties of the aircraft or other structure. GVT data are used to validate analytical vibration and flight control models by measuring flight control transfer functions and structural frequency response functions. GVT can be performed by supporting a structure on a support system consisting of a complex arrangement of air shocks and supports, as disclosed, for example, in U.S. Pat. No. 6,619,127 B2 issued to Miller et al., and U.S. Pat. No. 6,422,511 B1 issued to Kalisz. More primitively, GVT may be performed on an aircraft by simply reducing the air pressure in the tires. Reducing the air pressure in the tires, however, may not be the most effective method of testing because the tire construction creates a non-linear spring and the influence is difficult to extract from the results. Similarly, other GVT techniques may produce results, which are contaminated by a series of unknown non-linearities, as well as by test support system non-linearities.
Ideal dynamic testing conditions minimize or eliminate risks to test data associated with test support systems. For example, structures may be tested with fixtures designed to impose a set of boundary conditions on the system, which do not significantly alter the measured data. Finding a suitable fixture with negligible effects on the dynamics of the test structure is often a challenge, especially since most fixtures require a large amount of mass and isolation when connecting a test article. At least one known test method utilizes bungee cords in an effort to provide for a cost-effective fixture. Though the cords may be inexpensive and easy to work with, they are apt to change length and creep over time and they exhibit more damping than an ideal spring. Therefore, although desirable results have been achieved using prior art support systems for dynamic testing of structures, there is room for improvement.